Conventional spacecraft designs attempt to minimize the amount of angular momentum build up and therefore minimize the size of the momentum storage system and the number of momentum dumping operations required to dispose of any momentum build up. Spacecraft with larger momentum storage systems are heavier than those with smaller storage systems. Further, momentum dumping operations require utilization of fuel resources and thus increase the operational costs of the spacecraft and its complexity.
These spacecraft designs also use protective devices, such as thermal blankets, on the large side-mounted reflectors to provide thermal protection for the reflectors. Conventional thermal blanket designs utilize carbon-loaded (black) blankets on the back side of the reflectors and a Germanium-coated (reflective) sunshield on the front surface of the reflector. For a spacecraft with the same reflector design on both sides of the spacecraft, the surface properties of each one of the pair of reflectors are symmetric.
FIG. 1 illustrates an example of a conventional spacecraft 10 having a spacecraft body 12 and symmetric right and left reflectors 14, 16. Each of the reflectors 14, 16 are dual surface antennas that have a sunshield on a respective front surface 18 and a black reflective blanket on a respective back surface 20. As the spacecraft 10 rotates throughout a twenty-four hour period, sunlight illuminates all sides of the spacecraft 10. The solar pressure acting on the spacecraft 10 is larger on the reflective front surfaces 18 than the black back surfaces 20. As the designs of the reflectors 14, 16 are symmetric, the solar-induced torque on the spacecraft 10 is balanced as it rotates over a twenty-four hour period. As the solar pressure is balanced, the resulting net angular momentum build up about the y-axis is zero.
For a spacecraft with asymmetric reflector designs, such as a bare graphite left reflector (black on both surfaces) and a blanketed right reflector (reflective front surface and black back surface), the solar pressure on the reflective front surface of the right antenna is greater than on the black front surface of the left antenna. As the spacecraft rotates over a twenty-four hour period, there is a net build up of angular momentum due to the unbalanced pressure on the front side of the left reflector. With this configuration, the angular momentum can quickly exceed the momentum storage capability of the spacecraft thus requiring more frequent angular momentum dumping operations on-orbit.